A normal sound reproducing apparatus can directly emit a sound wave of an audible band into a medium such as air through a diaphragm, to propagate the sound wave of the audible band in a relatively broad range by a diffraction effect.
As opposed to this, a sound reproducing apparatus with high directivity has been put into practice for selectively propagating the sound wave of the audible band only to a specific space range. This sound reproducing apparatus is generally called a super directional loudspeaker or a parametric loudspeaker. This modulates a signal in the audible band with a signal in an ultrasonic band as a carrier, further amplifies the signal by a specific scaling factor, and thereafter inputs this modulated signal into a sound emitting unit made up of an ultrasonic transducer and the like, to emit the signal as a sound wave of the ultrasonic band into the medium such as air.
The sound wave emitted from the sound emitting unit propagates to the medium with high directivity due to a propagation characteristic of the ultrasonic wave as the carrier. Moreover, during propagation of the sound wave of the ultrasonic band in the medium, with the medium having elastic nonlinearity, an amplitude of the sound wave of the audible band accumulatively increases, while the sound wave of the ultrasonic band attenuates since being absorbed by the medium or diffused over a spherical surface. As a consequence, the sound wave of the audible band, having been modulated to the ultrasonic band, is self-demodulated to the sound wave of the audible band due to the elastic nonlinearity of the medium, thereby to allow reproduction of the sound wave of the audible band only in a restricted narrow space range.
That is, the super directional loudspeaker is one making use of the elastic nonlinearity of the medium where the sound wave propagates and the high directivity of the ultrasonic wave. For example, the use of the super directional loudspeaker as a loudspeaker for descriptions of exhibitions in an art museum or a museum allows transmission of a sound wave of an audible band only to a person present within a specific space range.
The foregoing sound reproducing apparatus uses, as a carrier frequency, a frequency in the vicinity of a resonance frequency for exciting a resonance mode of the ultrasonic transducer made up of a piezoelectric body and the like in order to increase sound pressure of the sound wave of the audible band which is reproduced by as small an input electric field as possible. In the vicinity of the resonance frequency, mechanical quality factor Qm (constant indicating sharpness of a mechanical vibration displacement in the vicinity of the resonance frequency at the time of the piezoelectric body or the like producing resonance vibration) is high, and a maximal vibration displacement can be obtained with respect to an alternating electric field that is applied.
However, there are variations in resonance frequency of the ultrasonic transducer between individuals, which is attributed to structural conditions such as shapes, dimensions and supporting and fixing methods of the piezoelectric body and the other constitutional elements, and is attributed to material characteristic conditions such as a piezoelectric constant and an elastic constant generated by such processes as polarization and sintering in the case of the piezoelectric body being ceramics. Further, mechanical quality factor Qm is also influenced by a temperature change of the ultrasonic transducer itself and load fluctuations due to the medium such as air, and there has thus been a problem in that, even when an electric fields with the same frequency and the same amplitude are applied to a plurality of ultrasonic transducers, respective vibration amplitudes of the ultrasonic transducers differ, and thereby at the time of demodulation and reproduction of the signal in the audible band, desired sound pressure cannot be obtained depending upon a frequency band of the signal in the audible band.
It is to be noted that Non-Patent Document 1 is known as prior art document information concerning the above sound reproducing apparatus.